1. Field of the Invention
The present invention relates to a hydraulic control circuit arrangement of a single-acting cylinder adapted to be used as, for example, a hydraulic load lift cylinder of a forklift truck, and more particularly, relates to a hydraulic control circuit arrangement provided with hydraulic directional control and pilot valves and capable of operating a common single-acting vertical cylinder as a ram cylinder for a low load, and as a piston cylinder for a high load.
2. Description of the Related Art
In general, forklift trucks use vertical load lifting cylinders to move up and down a lift member on which a load handling device is mounted, and U.S. Pat. No. 4,657,471 to Shinoda et al discloses a pair of separate load lifting cylinders disposed adjacent to a front upright assembly of the truck in such a manner that the two load lifting cylinders are laterally spaced apart to improve the forward view from the driver's seat of the forklift truck.
The operation of the load lifting cylinder is controlled by a hydraulic control circuit arrangement such as that disclosed in, for example, Japanese Unexamined (Kokai) Patent Application No. 57-134006. This known hydraulic control circuit arrangement of JUP-A-57-134006 is provided with a hydraulic pump and a control valve.
Other typical conventional hydraulic control circuit arrangements for controlling the operation of single-acting vertical cylinders are shown in the accompanying FIGS. 19 through 27, in which FIGS. 19 through 22 show a first type of such an arrangement in which a pilot operated valve 52 operable to switch the operation of a single-acting cylinder 53, e.g., a single-acting lift cylinder, from a ram type operation to a piston type operation, and vice versa, is independently arranged in a hydraulic circuit to connect the single-acting cylinder 53 and a manually operated directional control valve 51, and FIGS. 23 through 27 show a second type of such an arrangement in which a similar pilot operated valve 52 is built-in to a spool 51a of a manually operated directional control valve 51.
In the above first and second types of conventional hydraulic control circuit arrangements, when the directional control valve 51 (the other manually operated directional control valve 51a is arranged for controlling the operation of a non-illustrated single-acting cylinder) is shifted to a position at which a pump conduit 54 of a hydraulic pump P communicates with a bottom side conduit 56 of the single-acting cylinder 53, an operating oil from the hydraulic pump P is supplied to a bottom side chamber 58 of the cylinder 53 to thereby cause a lifting motion of the single-acting cylinder 53. Nevertheless, when a hydraulic pressure acting on the pilot-operated valve 52 from a pilot line 60 connected to the bottom side conduit 56 is lower than a set pressure of the pilot-operated valve 52, i.e., when the single-acting cylinder 53 is subjected to a light load, no lifting motion of the pilot-operated valve 52 occurs while maintaining the position thereof shown in FIG. 19 or FIG. 23. That is, as shown in FIG. 20 or 25, a rod side conduit 57 of the single-acting cylinder 53 is prevented by the pilot-operated valve 52 from communication with a tank conduit 55 of a hydraulic tank T, and as a result, an operating oil in a rod side chamber 59 of the single-acting cylinder 53 flows through a check valve 61 disposed in the piston of the single-acting cylinder 53 into the bottom side chamber 58. Accordingly, the cylinder 53 acts as a ram type cylinder having a pressure receiving area corresponding to the cross-sectional area of the piston rod having a diameter "d".
On the other hand, when the directional control valve 51 is shifted to connect the pump conduit 54 with the bottom side conduit 56 of the single-acting cylinder 53, and when the hydraulic pressure in the pilot line 60 is higher than the set pressure of the pilot-operated valve 52, i.e., when the single-acting cylinder 53 is subjected to a heavy load, the pilot pressure passing through an orifice 63 acts on a needle valve 62 of the pilot-operated valve 52 whereby the needle valve 62 is urged to an open position thereof. Accordingly, a pressure differential appears across the orifice 63 to shift a pilot spool 52a of the pilot-operated valve 52 from the position shown in FIG. 20 or 25 to a leftward position shown in FIG. 21 or 26. Accordingly, the rod side conduit 57 of the single-acting cylinder 53 is connected with the tank conduit 55 through a passage 64 of the pilot-operated valve 52, and therefore, the operating oil in the rod side chamber 59 of the single-acting cylinder 53 flows through the rod side conduit 57 and the tank conduit 55 toward the hydraulic tank T, and thus the single-acting cylinder 53 acts as a piston type cylinder having a pressure receiving area corresponding to the cross-sectional area of the piston having a diameter D thereof. When the single-acting cylinder 53, i.e., the lift cylinder, begins to act as the piston type cylinder, a hydraulic pressure exerted by the hydraulic pump P is temporarily lowered, and therefore, the needle valve 62 is shifted to return to a closed position thereof due to the lowering of the pressure of a pilot line 60. Nevertheless, when the pilot spool valve 52a of the pilot operated valve 52 is shifted to the open position thereof, whereat the rod side conduit 57 is communicated with the tank side conduit 55, the pilot line 60 communicates with the tank conduit 55 through a passage 65 of the pilot-operated valve 52 to permit a flow of the pilot oil in the pilot line 60 through the orifice 63. Therefore, a pressure differential across the orifice 63 is maintained, and accordingly, the pilot spool 52a of the pilot-operated valve 52 is also maintained at the open position thereof until the directional control valve 51 is manually shifted to a neutral position.
When the directional control valve 51 is manually shifted to a position for connecting the bottom side conduit 56 of the single-acting cylinder 53 with the tank conduit 55 of the hydraulic tank T, the operating oil in the bottom side chamber 58 of the cylinder 53 is allowed to return to the tank T, and accordingly, a lowering motion of the single-acting lift cylinder 53 occurs to generate a negative pressure condition in the rod side chamber 59 of the lift cylinder 53. At this stage, an orifice or choke 66 disposed in the tank conduit 55 generates a rise in the pressure in the tank conduit 55, and as a result, a pressure differential appears between the rod side chamber 59 of the single-acting cylinder 53 and the tank conduit 55, due to the negative pressure in the chamber 59 and the pressure rise in the tank conduit 55, and a flow of an operating oil in the tank conduit 55 having a rising pressure into the rod side chamber 59 of the single-acting cylinder 53 is allowed by a forcible opening of a check valve 67 disposed in the pilot-operated valve 52 as shown in FIG. 22 of the first type control circuit arrangement, and therefore, the lowering motion of the cylinder 53 occurs.
In the second type control circuit arrangement, as shown in FIG. 27, an operating oil in the bottom side conduit 56 of the single-acting cylinder 53 flows into the rod side chamber 59 of the cylinder 53 via a tank port of the directional control valve 51 and the rod side conduit 57, and therefore, the lowering motion of the cylinder 53 occurs.
In the above-described conventional first and second types of hydraulic control circuit arrangements for the single-acting lift cylinder 53, the orifice or choke 66 must be provided in the tank conduit 55, to allow a flow of the operating oil from the bottom side conduit 56 to the rod side chamber 59 of the lift cylinder 53, and thus compensate for an expansion of the rod side chamber 59 which occurs during a lowering of the cylinder 53. Nevertheless, the orifice or choke 66 in the tank conduit 55 brings the following defect. Namely, when the hydraulic pump P is operated, even if the single-acting lift cylinder 53 is not operated, a given amount of an operating oil flows from the hydraulic pump P into the hydraulic tank T through the orifice or choke 66, and therefore, a constant load is applied by the orifice 66 to the hydraulic pump P. Accordingly, a loss of an hydraulic energy as well as a heating of the operating oil occur, due to the existence of the orifice or choke 66 in the tank conduit 55.
Also, in the hydraulic control circuit arrangement for the single-acting lift cylinder, the rod side conduit 57 must have a large diameter. This is because the operating oil must always flow smoothly into the rod side chamber 59 through the rod side conduit 57, under a lowest possible flow resistance. But when the single-acting lift cylinders are arranged in a forklift truck, the rod side conduits 57 must be disposed to run along the upright masts of the truck, and therefore, if these conduits 57 are made of pipes having a large diameter, the forward view from a driver seat of the forklift truck is obstructed.
In addition, in the first type hydraulic control circuit arrangement shown in FIG. 19, when the single-acting lift cylinder 53 is operated to act as a piston type cylinder for supporting a given load from the underside, the pilot line 60 is held in communication with the tank conduit 55 through the passage 65 of the pilot-operated valve 52. Accordingly, an operating oil in the bottom side conduit 56 of the cylinder 53 gradually leaks into the tank conduit 55 through the pilot line 60 and the passage 65, and therefore, an unfavorable gradual lowering of the lift cylinder 53 occurs due to the force of gravity. Furthermore, such a gradual lowering of the lift cylinder 53 causes a gradual expansion of the rod side chamber 59 of the single-acting lift cylinder 53, without compensation, and thus it is filled by an introduction of the operating oil. As a result, when the lift cylinder 53 is subsequently operated to act as a ram type cylinder, the cylinder 53 initially acts as a piston type cylinder before acting as a ram cylinder. Thus such a time lag occurs before the start of the ram cylinder operation.